1. Field
This invention relates to information busses, and more specifically to insuring the coherency of independent busses.
2. Background
Most processing systems have, at a minimum, basic building block units of a processing unit, memory, and Input/Output (I/O) devices. Some may also include a memory controller that provides decode and control functions for accesses to and from memory. The I/O devices may be managed by an input/output (I/O) controller. An I/O controller is useful when there are multiple I/O devices of various types. Generally, only one I/O device, or other device, may access memory at a time.
FIG. 1 shows a block diagram of an example computing system. The computing system includes a memory controller 10, graphics device 12 (e.g., display), processing unit 14, memory device(s) 16, and I/O controller 18. Memory controller 10 includes a memory 22 for buffering data between graphics device 12 and memory 16, a memory 24 for buffering data between I/O controller 18 and memory 16, and a controller 11 that manages data written to and read from memory 16. Memories 22 and 24 may be first-in/first-out (FIFO) memories. I/O controller 18 includes a memory 25 that buffers data between memory controller 10 and I/O controller 18, a Peripheral Component Interconnect (PCI) interface 32 that interfaces to a PCI bus 34, and direct memory access (DMA) controllers 26, 28, 30 and 31 that control various I/O devices. Memory controller 10 and I/O controller 18 are connected by bus 20.
In the system architecture shown in FIG. 1, if software (in processing unit 14) needs to configure a DMA controller device (e.g., Universal Serial Bus (USB), Integrated Drive Electronics (IDE)) which is a legacy DMA controller, the software will send out a configuration cycle through bus 20, target the DMA controller device, and have the cycle return through bus 20 to the processing unit. However, for this to occur, the software will have to be given specific information that the DMA device resides on the port to the bus that provides the DMA operations. Typically, this may be accomplished through defining a PCI-PCI bridge such that bus numbers can be defined. However, in order for this to work, software changes may be required because current DMA controller drivers which talk to DMA devices, such as IDE, may not work behind a PCI bridge under certain operating systems (e.g., Windows 98).
Another disadvantage of the architecture according to FIG. 1 is that sending processing unit configuration cycles across one bus (e.g., bus 20) may incorporate a high overhead. The data size may be small and may interrupt high-speed data transfers from DMA controller operations, thus lowering overall system performance.
Moreover, the architecture of FIG. 1 becomes problematic as more DMA controller devices need to be added to I/O controller 18. In the architecture in FIG. 1, data and other information (e.g., configuration data from processing unit 14) passes through one bus 20. As more DMA controllers are added to the I/O controller, the bandwidth needed out of the port on I/O controller 18 driving bus 20 needs to be increased. However, bus 20 may only be capable of handling up to a certain amount of bandwidth since bus 20 may be limited by items such as: clock frequency, layout constraints on the motherboard, cost of the motherboard, arbitration issues, etc.
More buses may be added to help address this problem. However, when more ports are added (for additional buses), a new issue arises as to which bus processing unit 14 uses to read the status of each DMA controller device. In the architecture in FIG. 1, processing unit 14 always sends instructions and requests through one bus (e.g., bus 20). However, when multiple ports and buses exist, the processing unit must know which port and bus to use to access a particular DMA controller device. If a DMA controller uses one bus for access memory, and the processing unit uses a different bus for reading status of the memory access from the DMA controller, a coherency issue between the buses arises. That is, for example, the processing unit may be unsure as to whether data has been actually written to memory (and not sitting in FIFO 24) when the DMA controller send status back to processing unit 14 that the DMA operation (read) has been completed.